Method for dosing an urea based reducing agent into a gas exhaust stream

ABSTRACT

The present invention provides for a method for dosing an urea-based reducing agent into a exhaust gas stream generated from a combustion engine and addressed to an aftertreatment system, e.g. SCR or SCRT system. According to the method of the invention the gas exhaust stream is conveyed into a dosing housing which develops along a longitudinal axis. In particular the gas is conveyed by generating an annular inlet jet inclined with respect of said axis. The urea-based reducing agent is injected by generating, inside the housing, an urea-based reducing agent spray which is preferably coaxial to said axis of the housing.

FIELD OF THE INVENTION

The present invention relates to a method for dosing an urea-basedreducing agent (e.g. a solution of urea in water) into a gas exhauststream generated from a combustion engine and addressed to anaftertreatment system (e.g SCR or SCRT device). The method according tothe invention allows to improve the mixing of the reducing agent intogas exhaust stream and consequently to improve the catalyst efficiencyof the aftertreatment device.

DESCRIPTION OF THE PRIOR ART

As it is know, a problem in the field of internal combustion engines, inparticular with diesel engines, whether turbocharged or not, is theformation of nitrogen oxides during combustion. The nitrogen oxides aredischarged with the engine exhaust gases and represent one of the mainpollutants. In order to reduce the emissions of nitrogen oxidesapproximately up to 90%, selective catalyst reduction (SCR) devices havebeen developed. Depending on the particulate emission limits, thesesystems can be equipped with a particulate trap (SCRT System).

The functioning of SCR and SCRT device is based on the reaction,promoted by an appropriate catalytic unit, between the nitrogen oxidesin the exhaust gases and ammonia specifically introduced as reducingagent. The ammonia is usually introduced in the form of a preferablyliquid reagent able to release ammonia, under suitable temperatureconditions or by the action of specific catalysts. The preferred sourceis usually urea in an aqueous solution, for example between 10 and 60%in weight, from which the ammonia is obtained by hydrolysis.

The urea is generally nebulized in a dosing module which is locatedupstream the SCR-SCRT system. FIG. 1 and FIG. 2 are examples ofconventional arrangements for a dosing module. In particular, FIG. 1shows a portion of an exhaust gas line comprising an SCR catalyst, adosing module and a mixing device interposed between the dosing moduleand SCR catalyst. The mixing device has the function to promote andimprove the mixing. The exhaust gas stream, coming from the engine, isaxially introduced into the dosing module and the urea-solution issprayed in the exhaust gas by an injector placed on the centerline(axis) of the housing of the dosing module. In the known solution showsin FIG. 2, urea-water solution is instead introduced in the dosingmodule by an injector inclined with respect to the direction of theexhaust gas stream. In other words, in the solution in FIG. 2, thereducing agent is injected laterally from a portion of the wall of thehousing of the dosing module.

In FIG. 1 also reactions regarding urea-base reducing agent (e.g.urea-water solution) are indicated. After the atomization of saidsolution by spraying, the evaporation of water starts according to thereaction:(NH₂)₂CO[aqueous]→(NH₂)₂CO[solid]+6.9H₂O[gas]

After the evaporation of water the urea decomposition starts accordingto reactions:(NH₂)₂CO[solid]→NH₃[gas]+HNCO[gas]HNCO[gas]+H₂O→NH₃[gas]+CO₂+[gas]

The injection methods proposed by the solutions in FIGS. 1 and 2 havefound to involve many drawbacks. In particular said methods do not allowa complete decomposition of urea (reactions relative to phase 3 inFIG. 1) and a uniform mixing of ammonia (NH₃[gas]) with the exhaust gas(CO₂[gas]). A non-uniform mixing disadvantageously reduces theefficiency of the SCR system.

In the solution shown in FIG. 1, incomplete decomposition of urea is dueto the fact that the spray droplet size is fixed by nozzlecharacteristic and that the exhaust gas stream is axially introducedinside the dosing module housing. Consequently, after the atomization(phase 1) no further aerodynamic droplet break up occurs. Instead, inthe solution of FIG. 2, the asymmetric installation of the injectorgenerates an irregular urea-water spray distribution in the dosingmodule housing thus reducing the maximal possible NOx conversion rate.

Also it has to be noted that decomposition of urea-water solution, maycause formation of other products in particular isocyanic acid. This isan highly reactive compound that tends to form liquid deposits, such asliquid films, or solid deposits on the various parts of the exhaustsystem (e.g. pipes, deflectors, SCR-SCRT System). This is caused bycontacting the reacting agent solution with cold surfaces such as forexample the walls of the dosing module housing or those of the gasexhaust pipe.

The arrangements proposed in FIGS. 1 and 2 as well as the others knownin the art, show an inconvenient strong spray-wall interaction.Consequently, known solutions do not allow to avoid the formation ofliquid deposits on the sidewall of the dosing module housing.

SUMMARY OF THE INVENTION

Therefore it is the main object of the present invention to provide amethod for dosing a reducing agent into a gas exhaust stream generatedfrom a combustion engine and addressed to a aftertreatment system whichallows to overcome the above mentioned problems/drawbacks.

Within this aim, a first object of the present invention is to provide amethod for dosing a reducing agent into an exhaust gas stream whichallows a complete urea decomposition and an uniform mixing of ammoniawith the exhaust gas.

Another object of the present invention is to provide a method fordosing a reducing agent into an exhaust gas stream which avoids theinteraction downstream the injection position between the reducing agentand cold walls of the exhaust gas system (e.g. dosing module walls,exhaust gas walls, aftertreatment system walls).

Not the last object of the present invention is to provide a methodwhich is highly reliable and relatively easy to perform at competitivecosts.

These and further objects are achieved by a method as described in theattached claims which form an integral part of the present description.In particular according to the method of the invention, said exhaust gasis conveyed into an dosing module housing which develops along alongitudinal axis. In particular the exhaust gas is conveyed bygenerating an annular inlet jet inclined with respect to the axis of thedosing housing. Moreover according to the method of the invention, theurea-based reducing agent is dosed by generating an urea-based reducingagent spray which is preferably coaxial to said axis of the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become fully clear from the following detaileddescription, given by way of a mere illustrative and non limitingexample, to be read with reference to the attached drawing Figures,wherein:

FIGS. 1 and 2 show conventional arrangements of a dosing housing usedfor dosing an urea based reducing agent in a dosing module crossed by anexhaust gas stream;

FIGS. 3 and 4 show schematically a first arrangement for a dosing modulewhich allows to perform the method according to the invention.

FIG. 5 shows schematically a second arrangement for a dosing modulewhich allows to perform the method according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method for dosing an urea-basedreducing agent into an exhaust gas stream generated from a combustionengine, for example a diesel engine. For the purposes of the presentinvention, by the expression “urea based reducing agent” it is meant anurea-based solution, e.g. water-urea solution, able to develop ammoniawhen injected in an exhaust gas current addressed to a after-treatmentdevice such as, for example, an SCR or SCRT device.

The method according to the invention comprises a first step ofproviding a dosing module having a dosing housing 20 which developsalong an axis X (also indicated with longitudinal axis X). In thisregard, FIG. 3 shows an exhaust gas system of a combustion engineprovided with an exhaust pipe 4 and a dosing module which is locatedupstream to a mixing device 25. The latter is in its turn locatedupstream a catalytic device 30 such as for example a SCR or a SCRTdevice. As illustrated the dosing housing 20 can be a part of theexhaust pipe 4 and can have preferably a circular cross section. For thepurpose of the invention, by the expression “cross section” it is meanta section perpendicular to the axis X of the dosing housing 20.

According to the method of the invention, the exhaust gas stream comingfrom the engine is conveyed into said dosing housing 20 by generating anannular inlet jet (indicated with AJ) inclined with respect to the axisX. In other words, the exhaust gas is annularly introduced into thedosing module housing according to a inlet direction Y inclined to saidaxis X. In this way, the inlet exhaust gas stream presents a radialcomponent perpendicular to the longitudinal axis X and an axialcomponent which is parallel to the longitudinal axis itself.

The method according to the invention also provides for dosing saidurea-based reducing agent by generating a urea-based reducing agentspray (indicated with UWS) which is internal to the dosing housing 20and preferably coaxial to the axis X of the housing itself. More indetail, the reducing agent is sprayed, by means of spraying means, sothat the opening cone of the spray UWS is incident to the inletdirection Y of the annular inlet jet AJ of exhaust gas. As shown inFIGS. 3 and 4, the urea-based reducing agent spray UWS is generated sothat spray cone opening angle faces the angle α between the inletdirection Y of said annular jet AJ and the axis X of said dosing housing20.

FIG. 4 shows in detail the annular region of the dosing housing 20 wheresprayed reducing agent is mixed to the annular inlet jet AJ. In thisregion, a strong turbulence is generated. This turbulence increases thedroplet evaporation of the urea-based reacting agent and consequentlyincreases the following urea particle decomposition. In this way,reaction rates are advantageously increased.

With reference again to FIG. 4, the annular inlet jet AJ allows toprevent the droplets of the spray from hitting the sidewalls 20B of thedosing housing 20. In fact, the spray droplets directed towards thesidewalls 20B are deflected inward the dosing housing 20 due to theinlet direction Y of the annular inlet jet AJ. In this way, the spraydroplets can be flown in a central space (indicated with reference CS)of the dosing housing 20 without contacting the sidewalls 20B. In FIG. 4said central space CS is schematized by the dashed lines L1. Thediametral extension (indicated of D1) of this central space CS dependson the inlet speed of the annular jet AJ.

As shown in FIG. 4, the annular jet AJ is preferably conveyed by meansof an annular inlet 9 communicating to the dosing housing 20 inproximity of an end transversal wall 18. Spray injection meanspreferably comprises a spray nozzle 55 placed on the center of the endwall 18. According to the invention the inlet jet AJ is inclined withrespect to the longitudinal axis X of the dosing housing 20 of an angleα comprises between 30 and 150 degrees. In particular greatlysignificant results have been observed when said angle α is comprisedbetween 30 and 90 degrees and when the reducing agent spray has anhalf-cone with an opening angle β comprised between 5 and 40 degrees.

FIG. 5 shows an alternative arrangement of a dosing module in which theannular inlet jet AJ is inclined with respect to the longitudinal axis Xof the dosing housing 20 of an angle α greater than 90 degrees. Inparticular it has been observed that this arrangement advantageouslyavoids deposits near the nozzle 55.

It has been shown that the present invention achieves the aim and theobjects as set forth above. More in detail, it has been shown that themethod for dosing an urea-based reducing agent allows a completedecomposition and an uniform mixing of ammonia with the exhaust gas.Moreover, the method also avoids the formation of liquid deposit on theinternal surface of the dosing housing and of the gas exhaust pipe.

Many changes, modifications, variations and other uses and applicationsof the subject invention will become apparent to those skilled in theart after considering the specification and the accompanying drawingswhich disclose preferred embodiments thereof. All such changes,modifications, variations and other uses and applications which do notdepart from the spirit and scope of the invention are deemed to becovered by this invention.

Further implementation details will not be described, as the man skilledin the art is able to carry out the invention starting from the teachingof the above description.

The invention claimed is:
 1. Method for dosing a urea-based reducingagent into a gas exhaust stream generated by a combustion engine andaddressed to an after-treatment system (30), said method comprises thesteps of: providing a dosing module having a dosing housing (20);conveying said gas exhaust stream into said dosing housing (20); anddosing said urea-based reducing agent by generating, inside said dosinghousing (20), an urea-based reducing agent spray (UWS), characterizingin that said dosing housing (20) develops along an axis (X) and in thatsaid gas exhaust stream is annularly introduced into said dosing housing(20) by defining an axially symmetric annular inlet jet (AJ) inclinedwith respect to said axis (X) of said dosing housing (20), wherein theannular inlet jet (AJ) is axially symmetrical about the axis (X) alongwhich the dosing housing (20) develops.
 2. Method according to claim 1,wherein said urea-based reducing agent spray (UWS) is generated so as tobe coaxial to said axis (X).
 3. Method according to claim 1, whereinsaid urea-based reducing agent spray (UWS) is generated by means ofinjection means comprising a nozzle (55) located inside said dosinghousing (20).
 4. Method according to claim 1, wherein said urea-basedreducing agent spray (UWS) is generated so as to have an opening coneincident to the inlet direction (Y) of said annular inlet jet (AJ). 5.Method according to claim 4, wherein said urea-based reducing agentspray (UWS) is generated so as to have a spray half-cone opening angle(β) that faces the angle (α) between the inlet direction (Y) of saidannular jet (AJ) and the axis (X) of said dosing housing (20).
 6. Methodaccording to claim 1, wherein said annular inlet jet (AJ) is inclinedwith respect to said axis (X) of said dosing housing (20) of an angle(α) comprised between 30 and 150 degrees.
 7. Method according to claim6, wherein said annular inlet jet (AJ) is inclined with respect to saidaxis (X) of said dosing housing (20) of an angle (α) comprised between30 and 90 degrees.
 8. Method according to claim 1, wherein saidurea-based reducing agent spray is generated so as to have a half-coneopening angle (β) comprised between 5 and 40 degrees.
 9. Methodaccording to claim 1, wherein said urea-based reducing agent is formedby a solution of urea in water.